This invention relates to a DC motor and more particularly to a compact, high output motor of this type.
Four-magnetic pole, four-brush, DC motors have been used in environments where a high output and small size is desirable such as a starter motor for a motorcycle. The four-magnetic pole, four-brush DC motor is one in which an armature constituting a rotor is journalled within a cylindrical yoke constituting a stator case. A commutator is provided at one end of the armature. Four arc-shaped magnets are attached on the inside wall of the stator case at regular intervals of 90xc2x0 with adhesive. The magnets are generally made of a ferrite-base magnetic material (with magnetic flux density of about 0.4 T). A rotor shaft is journalled within the stator. Coils are wound on a core provided on the rotor shaft to form the armature.
The core is comprised of a plurality of radial core pieces, and a winding is wound around the core pieces in a given order to form the coils. The commutator is comprised of contact pieces corresponding to the core pieces of the coils, provided at one end of the rotor shaft. In contact with the commutator are four brushes, and rotation of the commutator allows the brushes to contact the contact pieces in turn so that the coils are energized to rotate the armature.
In a motorcycle the spatial constraints are large and size reduction of the starter motor is requisite. The thickness of the ferrite-base magnets constituting the stator of a conventional starter motor has been limited to about 5-6 mm from the technical viewpoint of manufacturing.
In addition, in order to enhance the ease of starting the motorcycle engine, improvement in the starter motor output is desirable. Improvement in starter motor output requires increased intensity of the magnetic field of magnets (magnetic flux density) to generate higher energy. To do this, it is necessary to allow a large current to flow on the coil side, corresponding to the higher energy caused by the higher magnetic flux density of the magnets. The large current requires a thick winding of the coils. Conventionally, the diameter of the wire of the winding is about 0.9 mm or less. This size is limited because of the structural requirements such as core shape, and the requirements of facilitating the winding operation.
The output can be increased by utilizing a neodymium-base magnet made of a magnetic material containing neodymium (Nd) or its compounds as a high energy magnet. The magnetic flux density of a neodymium-base magnet is about 1.2 T.
However, if the volume of the magnet is simply increased to enhance magnetic force in order to enhance the motor output, the magnet becomes thicker, and the diameter of the machine increases. This obviously contradicts the requirement of size reduction of the motor. Alternatively, if neodymium-base high energy magnets are used, a sufficient output increase corresponding to the magnet material cannot be effected if the diameter of the winding of the coils is limited to 0.9 mm.
If high energy magnets are used particularly in a starter motor for a motorcycle, the motor is required to have a limited size range. That mean that a simple increase in the magnet thickness necessitates a smaller armature due to the limited outside diameter, effecting no large output. In this case, even if the output characteristic is simply represented by the thickness or the volume of the magnet, or the outside diameter of the armature as a parameter, the characteristic of the peak output cannot be realized because the shape and size of the yoke can not be selected properly.
Therefore, the shape and size of the magnet has been determined, based only on convenience or costs of manufacturing, and there has been a lack of obtaining the optimum output characteristic.
Therefore it is a principal object of this invention to provide a DC motor which is small in size and capable of effecting a higher output.
This object is achieved by determining the magnet size through a parameter by which the peak output characteristic corresponding to the shape and size of the yoke can be recognized properly.
It is another object of this invention to provide a DC motor in which the output characteristic is based on a new parameter so that a size reduction and a high output can be effected.
This invention is adapted to be embodied in a DC motor comprised of a stator consisting of a plurality of arc-shaped magnets attached at circumferentially spaced, regular positions on an inside wall of a cylindrical yoke. A rotor shaft is journalled for rotation within the stator. An armature core is formed on the rotor shaft and is comprised of a plurality of radial core pieces and a winding wound around said core pieces to form the coils. The magnets are formed from a magnetic selected from a group of materials consisting of a magnetic material containing neodymium (Nd) and a ferrite-base magnetic material. The thickness of said magnets is represented by t and the outside diameter of said armature by d, the value of d/t is in the range of approximately 4-25.
In accordance with a further feature of the invention, the magnet material is a magnetic material containing neodymium (Nd) and the diameter of the winding is in the range of 1.0-2.5 mm and the value of d/t is in the range of approximately 8-25.
In accordance with another feature of the invention, the magnetic material is a ferrite-base magnetic material and the value of d/t is in the range of approximately 4-15 These relationships contrast with the prior art constructions where it has been found that in a conventional DC motor using ferrite-base magnets actual measurements of d/t of a conventional ferrite-base magnet DC motor for motorcycles is in the range of 2-3.